1. Field of the Invention
The present invention relates to a sensor signal output circuit and a method for adjusting the sensor signal output circuit, which are suitable, for example, for outputting an output signal of a temperature sensor to the outside with good linearity to temperature.
2. Description of the Background Art
In an electronic circuit apparatus using a heat generating device such as an IGBT or a power MOS-FET, the temperature of the heat generating device or its ambient environment is monitored by use of a temperature sensor. On this occasion, an output voltage of the sensor having a linear output characteristic with respect to temperature change, such as a temperature detecting diode, is converted into a PWM signal with a pulse width corresponding to the output voltage, and outputted to the outside for informing a monitoring circuit etc.
FIG. 3 shows an example of a typical configuration of such a sensor signal output circuit 1 in the background art. In the sensor signal output circuit 1, a voltage Vf generated in a temperature detecting diode 3 driven by a constant current source 2 is amplified by an operational amplifier 4. In the sensor signal output circuit 1, a triangular wave signal with a predetermined amplitude outputted by an oscillator 5 is compared with an output voltage of the operational amplifier 4 by a comparator 6 so as to generate a PWM signal with a pulse width corresponding to the output voltage. The sensor signal output circuit 1 is configured to output the PWM signal to the outside through a buffer amplifier 7.
Incidentally, as shown in FIG. 4, the operational amplifier 4 has an inverting input terminal to which an analog input signal V1 is inputted through an input resistor R1, and a non-inverting input terminal to which a reference voltage V2 divided by an input resistor R2 and a ground resistor R4 is inputted. In addition, a feedback resistor R3 is provided between the inverting input terminal and an output terminal of the operational amplifier 4. The operational amplifier 4 which constructs a differential amplifier circuit in this manner obtains an output voltage Vout as:Vout=(V2−V1)R3/R1which is fundamentally conditioned on [R1=R2] and [R3=R4].
Although the output voltage Vf of the temperature detecting diode 3 is linear with respect to the change of a temperature T, the output characteristic of the temperature detecting diode 3 contains a variation unique to the device. For this reason, the pulse width of the PWM signal may be displaced from a pulse width predetermined with respect to the temperature T, for example, as shown in FIG. 5A. Incidentally, in FIGS. 5A to 5C, the solid line A designates an output characteristic before correction and the broken line B designates a target output characteristic. The displacement of the pulse width of the PWM signal with respect to the temperature, that is, the displacement of the output characteristic is corrected in such a manner that an offset voltage and a gain of the operational amplifier 4 are adjusted to correct the output voltage of the operational amplifier 4, for example, as disclosed in JP-A-2008-5217.
The offset voltage of the operational amplifier 4 can be adjusted by changing the reference voltage V2 or by changing the ratio between the input resistor R2 and the ground resistor R4. In addition, the gain of the operational amplifier 4 can be adjusted by changing the ratio between the input resistor R1 and the feedback resistor R3. Specifically, first, as shown in FIG. 5B, the offset voltage of the operational amplifier 4 is adjusted in focus on a pulse width D1 of the PWM signal in a first temperature T1, so as to correct the output characteristic as designated by the solid line C. Then, as shown in FIG. 5C, the gain of the operational amplifier 4 is adjusted in focus on a pulse width D2 of the PWM signal in a second temperature T2 different from the first temperature T1, so as to correct the output characteristic as designated by the solid line D.
However, as described above, when the gain of the operational amplifier 4 is adjusted after the offset voltage of the operational amplifier 4 is adjusted, there is a problem that another new displacement appears in the pulse width D1 of the PWM signal in the first temperature T1, for example, as shown in FIG. 5C. That is, since the adjustment of the offset voltage and the adjustment of the gain on the operational amplifier 4 are performed separately from each other under independent parameters, it cannot be denied that the corrections of the output characteristic in the first and second temperatures T1 and T2 affect each other. Therefore, it is necessary to perform the adjustment of the offset voltage and the adjustment of the gain alternately and repeatedly in order to correct the output characteristic of the sensor signal output circuit 1 to the predetermined target output characteristic B.